Why is there such a thing as leap year?
People generally say there are 365 days in a year. By a year, I mean this is the time period it takes the earth to travel around the sun: 365 days.
Actually, however, it takes the Earth 365.25 days to make this trip. In other words, for every year we gain one-fourth of a day and every for years we gain an extra day. If nothing was done about this, our calendar would move backwards one full day every four years in relation to our seasons.